1. Field of the Invention
This invention relates to stabilization of 3-isothiazolone compounds present in the form of dilute aqueous solutions.
2. Description of the Prior Art
3-Isothiazolone compounds (hereafter "active ingredient" or AI) are a very important class of microbicides. Several species have been commercialized and are widely used to inhibit the growth of bacteria, fungi and algae. Among the most important species are 2-methyl-3-isothiazolone ("MI"), 5-chloro-2-methyl-3-isothiazolone ("CMI"), and especially mixtures thereof. A 3/1 weight ratio mixture of CMI and MI is used in a wide variety of commercial applications around the world. CMI is naturally unstable and much research has been devoted to stabilizing it in four separate classes of environments: (1) the isolated compound itself; (2) "concentrates" which are about 14 to 25% by weight aqueous solutions of the AI; (3) "dilute solutions" which are about 1 to 5% by weight aqueous solutions of the AI and which are designed to be diluted further when added to a locus; and (4) "use dilutions" which are the end use dilution in the locus to be protected and comprise substantially less than 1% by weight AI.
To stabilize the isolated compound (1), U.S. Pat. No. 4,150,026 teaches making metal salt complexes of isothiazolones. These complexes are taught to greatly enhance the thermal stability of solid isothiazolones, compared to the corresponding uncomplexed isothiazolones. No mention is made regarding the stabilization of aqueous solutions of isothiazolones, i. e., (2) or (3), or the use dilutions (4), against chemical decomposition of the AI.
U.S. Pat. No. 3,870,795 teaches the stabilization of 3-isothiazolone concentrates, i.e., (2), against chemical decomposition by addition of metal nitrite or metal nitrate. Among the useful nitrates are taught those of sodium, potassium, calcium, magnesium, ferric, ferrous, nickel, zinc, barium, manganese, silver, cobalt, and the like, and among the useful nitrites are taught those of sodium, potassium, calcium, magnesium, and the like. The '795 patent discloses that other common anions, including carbonates, sulfates, chlorates, perchlorates, and chlorides, are surprisingly ineffective. The '795 patent also teaches that the isothiazolone can be an aqueous solution of the compound itself, or of the complexes. According to '795, neither the concentrates, nor the complexes, nor non-aqueous 25% by weight solutions of either the compounds or the complexes, are stable unless stabilized with nitrates or nitrites. This patent does not teach stabilization of dilute solutions (3). All of the '795 examples are directed to 25% solutions of the compounds or the complexes thereof. The concentrates are thus taught by the prior art to be stabilized by nitrates or nitrites, irrespective of the metal counter ion, and the present invention is not inconsistent.
Commercial concentrates (2) contain 3:1 mixtures of CMI and MI as the active ingredient and magnesium nitrate stabilizer in an approximate 1:1 weight ratio of stabilizer to AI. Magnesium chloride is also present as a by-product of the neutralization step in the preparation of the isothiazolone.
To stabilize the AI in use dilutions (4), Law et al., U.S. Pat. No. 5,160,527 suggest metal salts (Cu, ferrous, ferric, zinc, manganese, magnesium) of an organic carboxylic acid of at least six carbon atoms, (which are water insoluble) or chelating anions selected from EDTA, 8-hydroxy-5-hydroxyquinolinate, gluconate, o-phenanthroline, quinolinate, N,N-bis(2-hydroxy-5-sulfobenzyl)glycine, lignosulfonate polymers, and polyacrylates. The use dilutions suggested by Law, et al., comprise loci which contain destabilizing components such as amines, reducing agents (e.g., bisulfites), and fuel. These "destabilizing components" react with the AI. Law et al. do not teach any method of stabilizing dilute solutions which consist of the AI and aqueous solvent in the absence of the aforementioned destabilizing components.
Willingham, et al., U.S. Pat. No. 5,118,699, teaches the stabilizing effect of hydrazide compounds for isothiazolones present in metal working fluids and similar loci containing components antagonistic to the AI. Willingham, et al., U.S. Pat. No. 5,142,058, teaches the stabilizing effect of alkyl halohydantoins and the like in similar antagonistic loci, i.e., use dilutions. These Willingham, et al., patents do not relate to stability in dilute solutions.
For dilute solutions (3), there are two conventional methods of stabilization. The first is to use high ratios (e.g., about 15:1) of Mg(NO.sub.3).sub.2 to AI. Mg(NO.sub.3).sub.2 is not effective at lower ratios, i.e., about 1:1. The second is to use a cupric salt as the stabilizer. The cupric salt is conventionally used at a ratio of about 1:10 to AI since only very small amounts of cupric ion are necessary for effective stabilization. The reason use of cupric ion is preferred in industrial situations is that too much nitrate salt is disadvantageous in certain applications or loci.
However, the cupric stabilizer has recently come under question because of government regulatory limits in certain countries on the amount permitted in water discharge streams.
Since dilute solutions have certain advantages over the concentrates (ease of handling, reduced potential for sensitization of workers, and greater flexibility of formulations), it became necessary to find an alternative stabilization method to the cupric ion or high levels of nitrates.